Oscillators



March 4, 11958 A. COOPER 293259813 OSCILLATORS Filed April' 1; L955 OSCILLATORS Albert Cooper, Beckenham, England, assignor to Muirhead & Co. Limited, Beckenham, England Application April 1, 1%5', Serial No. 498,689

Claims priority, application Great Britain March 7, 1955 3 Claims. (Cl. 25036) This invention relates to low-frequency oscillators and more particularly to oscillators having a high degree of negative feedback.

Oscillators of this type may be made to cover a very wide band of frequencies, ranging from a few cycles per second to the lower radio frequencies, and may be employed for a wide variety of purposes. To provide maximum utility it is, however, essential that the oscillator shall have a high degree of frequency stability and that the output shall be constant and substantially free from harmonics. These characteristics must be maintained despite variations in supply mains voltage or frequency, ambient temperature, or valve characteristics due to ageing and other detrimental factors.

The object of the present invention is to provide an oscillator of the type referred to having a very high degree of frequency stability, constancy and purity of output despite variations in operating conditions.

The invention consists of an oscillator in which the resistance-capacitance or resistance-inductance networks which determine the frequency of oscillation are contained in two arms of a four-arm bridge, the third arm of the bridge containing a resistance element having a high positive temperature coefficient of resistance and serving more particularly to provide frequency stability, and the fourth arm of the bridge containing a resistance element having a high negative temperature coefficient of resistance and serving more particularly to provide amplitude stability.

One or both of the resistances having high temperature coefiicient of resistance is/are heated by the output of a power amplifying valve responsive to the amplitude of oscillation.

The invention will be further described with reference to the accompanying drawings in which:

Figure 1 shows a simplified circuit diagram of an oscillator according to the invention.

Figure 2 shows a modification of part of Figure 1.

Figure 1 shows a circuit diagram of an oscillator in which the oscillation frequency is controlled and stabilized by a bridge circuit having four arms, generally designated l, 2, 3 and 4. Arms l and 2 contain resistancecapacitance networks Rl/Cl and R2/C2 respectively which determine the frequency of oscillation. Arm 3 contains a resistance element having a high positive temperature coefficient of resistance, for example a lamp RP. Arm 4 contains a resistance element RN having a large negative temperature coefficient of resistance and may conveniently consist of a resistor of the type known as a thermistor, having a heating element by which the temperature of the resistance element may be varied. The junction of arms 1 and 2 is connected to the grid of valve V1 and the diagonally opposite junction of arms 3 and 4 is connected to its cathode. Valve V1 is an amplifying valve with load resistance R3 in its anode circuit and is coupled to another amplifying valve V2 by capacitor C3, R4 being the grid resistance and R5 the bias resistance.

hired States Patent reduce amplification.

Valve V2 has load resistance R6 in its anode circuit and is coupled to valve V3 by capacitor C4, R7 being the grid resistance.

Valve V3 is a cathode follower, having bias resistance R8 and load resistance R9 and the output from the cathode is taken through capacitor C5 to potentiometer VRI, the slider of which forms the output connection. The top and bottom ends of VRl. are connected respectively to the junctions of arms 1 and 4 and arms 2 and 3 of the bridge net-work to provide full positive feedback to maintain oscillation.

A power amplifying valve V4 has its grid connected to the grid of valve V1 and provides an output proportional to the amplitude of oscillation. This output is fed through transformer TRl to the heater of the thermistor or the like, so that resistance RN is heated to a temperature corresponding to the amplitude of oscillation.

The steady anode current of valve V1 flows through the resistance RP and serves to raise its temperature to a suitable operating point. The oscillatory current fed back from the top of potentiometer VR1 to the junctions of arms 1 and 4 of the bridge also flows through resistance RN and resistance RP, thus providing negative feedback which varies in accordance with the amplitude of oscillation and acts to oppose any variation in the amplitude of oscillation.

By including amplifying valve V4 to operate the thermistor heater it is possible to produce a large reduction in resistance RN for a small increase in oscillation amplitude. This allows a correspondingly heavy oscillatory current to flow through resistance RP to raise its temperature and hence increase its resistance. By these means a small increase in oscillation amplitude produces a relatively heavy increase in negative feedback acting to The inclusion of V4 provides a further advantage in that any variation in the characteristics of resistance RN is automatically compensated by a change in the output of V4. V4 may also be arranged to heat resistance RP, thus further increasing its stabilizing effect. Figure 2 shows the input circuit to V1, including such a heating arrangement, the references to the elements being the same as those in Figure 1.

In the arrangement according to the invention, resistance RP serves another purpose. If, due to a change in mains voltage or from other causes, the high tension voltage of the oscillator changes, it will cause a change in the anode current of V1. This causes a corresponding change in the temperature and hence the resistance of element RP. An upward change in the high tension voltage will result in an increase in anode current through V1, producing an increase in the resistance of RP and, in turn, a rise in cathode voltage opposing the change in anode current. This cycle of events is reversed in case of a fall in high tension voltage. A change in anode current in V1 tends to produce a change in oscillation frequency and it has been found by experiment that RP serves more particularly to promote frequency stability and RN serves more particularly to promote amplitude stability.

There is a further feature of the invention which assists in promoting high stability. Resistance elements RN and RP in series are directly in parallel with output potentiometer VRl and the frequency determining network Rl/Ci. and R2/C2. A change resulting in a decrease in the resistance of RN also produces an increase in the resistance of RP and the total impedance in parallel with the output and input circuits remains substantially unchanged. To this end the amplification given by V4 may, of course, be adjusted by variation of its bias resistance R10.

Another feature of the invention results from the use of amplifying valve V4 to provide power to operate the heater of resistance RN, in that a small oscillation amplitudeis sufi'icient to enable V4 to provide adequate power. The oscillator may therefore be operated at a low amplitude of oscillation, which makes it possible to keep the harmonic content to a very low level.

-In a practical test of an oscillator according to the invention, :it was found that the output had a high degree of frequency stability, constancy of amplitude and freedom from harmonics over a very large frequencyrange, i.'e. 10'to 30,000-cycles per second. The variation in amplitude between 20 cycles per second and 30,000 cycles per second did not-exceed 5%, whilst within the narrower range of 50 cycles per second to 10,000 cycles per second the amplitude of oscillation was constant to within 1%. The total harmonic content or impurity between 30 cycles per second and 30,000 cycles per second did not exceed 0.2 and each individual'second or third harmonicdid not-exceed 0.1%. With the oscillator correctly adjusted for a mains power supply of 200 v., 50 cycles per second, a reductionof 10% inmains voltageproduced a frequency error of 0.007% 1 at 1,000 cycles per second and an error of 0.08% at 30,000 cycles per second, whilst an increase of 10% in mains voltage produced an error of 0.005% at 1,000 cycles per-second and an error of 0.04% at 30,000 cycles per second. '-An increase or decrease of 5% in mains frequency producedan error which was too small to berecorded with accuracy.

The manner in which resistance-inductance networks may 'be substituted for the resistance-capacitance networks shown in'Figures 1 and 2 will be evident to those skilled in the art. It will also be clear that variations may bemade in the oscillator as described above without departing from the scope of the invention.

I claim: 7

1. Anoscillator comprising a maintaining amplifierand a four-arm bridge in which the first and second arms contain frequency-determining networks, the third arm contains a temperature-sensitive resistance having a high positive temperature coefficient ofresistance, and the fourth arm contains a temperature-sensitive resistance having a high negative temperature coefiicient of resistance, characterized in that both said temperature-sensitive resistances are also indirectly heated by the power output of a power amplifying valve controlled by the voltage between the grid of the first valve of the maintaining amplifier and the common earth line.

2. 'An oscillator as claimed in claim 1, in which the frequency-determining networks comprise resistances and capacitances.

3. An oscillator comprising'a maintaining amplifier and a fourarm bridge in which the first and second arms contain frequency-determining networks, each said frequencydetermining network consisting of resistive and reactive elements, the third arm contains a temperature-sensitive resistance having a high positive temperature coefiicient of resistance, and-thefourth arm contains a temperaturesensitive resistance having -a'-high negative temperature coefiicient of resistance, both said temperature-sensitive resistances being alsoindirectly'heated by the power output of a power amplifying valve-controlled by the voltage between the grid of the first valve of said maintaining amplifier and the common earth line.

Decade Oscillator,--by Edwards in Proceedings of the I. R. E., March 1951,-pages 277-278.

fPrecise Decade Oscillator, by Young in Bell Laboratories Record, November 1950, pages 487-489. 

